Four years ago we attempted to isolate a protein, produced by platelets, which causes histamine release from human basophils. R. Fisher et al., J. Allergy Clin. Immunol. 79, 196 (1987). This platelet-derived histamine releasing factor (PD-HRF) was present in supernatants obtained from washed platelets, and the supernatant was found to induce allergic skin reactions in human asthma patients. M. Weiss et al., J. J. Allergy Clin. Immunol. 81, 224 (1988). In order to further define its role in asthma, PD-HRF supernatants were used to challenge normal or allergic asthmatic rabbits by inhalation challenge. While normal rabbits in general were unaffected, asthmatic rabbits developed the typical changes associated with an asthma attack including early and late airway obstruction and increased hyperresponsiveness. W. Metzger et al. J. Allergy Clin. Immunol. 85(1) (1990); R. Fisher et al. J. Allergy Clin. Immunol. 85(1) (1990).